This is a multicenter, randomized, double-blind, placebo-controlled, sequential dose-ranging study of the efficacy, safety, and pharmacokinetics of a nonpeptide vasopressin v2 receptor antagonist, opc 41061 for the treatment of hyponatremia secondary to congestive heart failure in hospitalized patients. Four treatment groups will be studied using 5, 10, 15 or 30 mg of opc-41061 or placebo.